1. Technical Field
The present invention relates generally to utilization of fly ashes and other by-products of coal combustion in the generation of electrical power by coal-fired power stations. The invention relates, more particularly, to a rapid-setting self-hardening flowable backfilling composition which contains coal combustion by-products and method of utilizing same which permits backfilling of excavated utility trenches, the composition having an early strength gain to withstand normal roadway traffic loads without undue settling.
2. Background Information
The backfilling of utility trenches in roadway surfaces, for example, trenches for gas, water, sewer and electrical conduits, has often created problems in the past wherein the roadway surfaces must be closed to traffic for considerable periods of time to allow the backfilling material to gain sufficient strength to withstand placement of a paving course overlay and subsequent exposure to normal roadway traffic loads. After the ordinary maintenance, repair, or installation of conduits under roadway surfaces, lengthy periods of time and special materials have been required for such backfilling of excavations prior to laying down a paving course and resuming normal traffic over such filled excavations. This is particularly true where the pavement or other wearing course must be placed over a backfilled conduit area to withstand heavy loads. Where a conduit line is installed using the open trench method of construction and where relatively poor soil conditions exist, the normal method of construction to obtain the desired compaction of the backfilled material has been to remove the original soil and replace it with sand, gravel, or the intermixed original material, to obtain the specified backfill condition to withstand traffic loads. In some cases, it has been necessary to backfill in relatively thin layers and then mechanically compact each such layer which has resulted in a very expensive method of construction and maintenance which is unduly time consuming.
Additional problems have been encountered where the soil bedding for the conduit has been so undesirable that it must be removed and replaced with adequate bedding material for the conduit. It is important to have proper bedding under conduits so that the resulting compressive forces are equalized throughout the length of the conduit especially under roadway surfaces where rigid pipe is utilized as the conduit material. If the bedding or backfill does not uniformly support the conduit throughout its length, the conduit may be sheared, ruptured or otherwise fail, due to uneven compressive loading.
Where utility trenches are backfilled, the use of concrete is not an adequate solution to provide proper support for the conduit or the overlying roadway surfaces. If the backfill material is concrete, it is extremely difficult and expensive to subsequently excavate through the concrete to either repair or replace the conduit, or install lateral connections to the conduit, as desired or required.
In the prior art, controlled density fill materials have been utilized which include a Portland cement as one of their constituents; however, such cement-containing compositions normally attain much greater strength than required and present severe problems upon reexcavation for subsequent maintenance, repair or modification of the conduits. Some such backfilling materials for conduit trenches have obtained compressive strengths in the range of 200 to 800 psi which make reexcavation extremely difficult and costly to perform. Existing flowable backfilling materials which have been designated as controlled-density fill materials do not have the normal structural strength of concrete and require more expensive constituents than the present invention to obtain desired compressive strengths which are less than conventional concrete.
The present invention utilizes primarily fly ashes which are by-products from coal-fired power stations which are relatively inexpensive and readily available. Some particular fly ashes exhibit cementitious properties which are valuable for the preparation of rapid-setting self-hardening backfill materials which are capable of meeting city street and state highway road specifications for backfill materials beneath roadways. Class C fly ash containing significant amounts of calcium oxide is particularly valuable for use in the subject invention to obtain high early strength which is readily controllable within desired limits and permit its ready use in roadway excavations which may be reexcavated as required without difficulty.